Current multiplication transistors



p 1958 R. w. LANDAUER 2,854,588

CURRENT MULTIPLICATION TRANSISTORS Filed Dec. 23. 1953 INVENTOR. ROLF W.LAN DAUER ATTORNE United States Patent 9 2,854,588 CURRENTMULTIPLICATION TRANSISTORS Rolf W. Landauer, Poughkeepsie, N. Y.,assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Application December 23, 1953, Serial No.399,996 4 Claims. (Cl. 307-885) This invention relates to transistors,especially to transistors of the current multiplication type, and tomethods of producing such transistors and controlling their currentgain.

The current multiplication or point contact type of transistor comprisesa small block of semi-conductive material, to which are applied at leastthree electrodes or contacts. Where only three electrodes are used, theyare respectively termed the base, collector and emitter electrodes. Thebase contact is conventionally ohmic, i. e., its resistance isindependent of the direction and magnitude of current flow. The basecontact usually has a substantial contact area. The emitter andcollector electrodes are point contacts and have rectifying orasymmetric impedance characteristics, i. e., their impedance isdifferent for opposite directions of current flow.

The semi-conductive material in the body may be either n-type or p-type.The n-type semi-conductive material contains impurities providing anexcess of electrons which are free to move for the carrying of electriccurrents, whereas p-type semi-conductive material contains impuritieswhich result in a smaller number of electrons than are present in thepure semi-conductive material. The spaces in the lattice structure whichin the pure material would be occupied by the missing electrons arecalled holes and can be considered to act like movable positivelycharged current carriers.

For most point contact transistors the semi-conductive material used isn-type germanium. When potentials are properly applied between the baseand each of the other two electrodes, a translating device is producedwherein variations in current in the input circuit (usually theemitter-base circuit) cause variations in current in the output circuit(usually the collector-base circuit).

There is disclosed and claimed in the copending application of RichardF. Rutz, Serial No. 354,955, filed May 14, 1953, entitled Method forImproving the Current Amplification of a Transistor, a transistor of thepoint contact type having a third point contact, which functions as asecond emitter electrode, providing an auxiliary reservoir of excesscurrent carriers. The second emitter electrode as disclosed by Rutzprovides certain substantial advantages in the transistorcharacteristics, particularly an increased current amplification factor.The transistor disclosed by Rutz is, however, subject to certainlimitations. One such limitation is that the spacing between the secondor auxiliary emitter and collector must be equal to or greater than thediffusion length for the average lifetime of the excess carriers in thesemi-conductor material. In order to reduce this necessary spacing, itis therefore desirable to construct transistors of the Rutz type fromshort lifetime germanium.

Another difficulty with the Rutz transistor is an undesirable decreasein the back resistance (resistance to current flowing from base tocollector) under certain conditions of operation. This decrease in theback resistance is noted particularly if any attempt is made to re-Patented Sept. 30, 1958 duce the spacing between the auxiliary emitterand collector electrode below the diflfusion length. It is highlydesirable, especially in transistors intended for use in switchingcircuits, that the back resistance be maintained at a high levelthroughout the transistor operation.

An object of the present invention is to provide an improved transistorhaving a high current amplification factor.

Another object is to provide an improved transistor of the typedescribed in which the high current amplification factor is secured by amechanism similar to that disclosed by Rutz, namely an auxiliaryreservoir of excess carriers.

Another object of the invention is to provide a transistor of the typedescribed in which the reservoir of excess carriers is controlled at alltimes.

The foregoing objects are attained by provision of a transistor havingtwo emitter electrodes and two collector electrodes. Only one of the twocollector electrodes serves as the output electrode of the transistor.The first emitter is more or less conventional, being preferably locatedvery near the output electrode. The second emitter is located at a pointsubstantially equidistant from the two collectors. The distributionbetween the collectors of carriers flowing through the second emitter iscontrolled by the flow of carriers from the first emitter. Thepotentials of the several electrodes may be controlled as desired toregulate the current amplification factor.

Other objects and advantages of the invention will become apparent froma consideration of the following specification and claims taken togetherwith the accompanyin g drawing.

In the drawing, the single figure represents, somewhat diagrammatically,a transistor embodying the invention and associated circuit elements.

Referring to the drawing, there is shown a transistor including a body 1of semi-conductive material having a base electrode 2, two emitterelectrodes 3e and 4e, and two collector electrodes 50 and 6c. The body 1is illustrated as being of n-type semi-conductive material, and thedirections of current flow and the various polarities correspond to therequirements of that material. Those skilled in the art will readilyrecognize that p-type material may alternatively be used, withconsequent reversal of the current directions and polarities.

The emitter electrode 3e is located mid-way between the collectorelectrodes 50 and Go. In other words, the distance x between emitter 3eand collector 5c is made substantially equal to the distance y betweenemitter 3e and collector 6c. While these three electrodes are preferablylocated in one straight line, as indicated in the drawing, it is onlynecessary that the distance x be substantially equal to the distance y.

The emitter electrode 4e is located between emitter 3e and collector 6c,and preferably very close to collector 6c, the spacing there beingsubstantially the same as between the emitter and collector inconventional transistors.

The base electrode 2 is connected to ground. The emitter electrode 3e isconnected to ground through a resistor 17 and a biasing battery 7. Theemitter electrode 4e is connected to ground through a signal generator 8which is shown by way of example as including an A. C. signal generator9 and a D. C. signal generator 10, indicated as a battery. The directionof current flow from emitter 4e into the body is the direction usuallyassociated with an emitter. Therefore the emitter 4e must be positivewith respect to the semiconductor material immediately adjacent, but itdoes not have to be positive with respect to the base 2. If collector 6cis negative and emitter 4e very close to it, it might be necessary tobias emitter 4e negatively with respect to the grounded base 2.

The collector electrode 50 is connected in series with a battery 11 anda resistor 12 to ground. The collector 6c is connected in series with aload resistor 13 and a battery 14, to ground. Output terminals 15 and 16are respectively connected to the collector 6c and to ground.

Operation In the following discussion, the operation of the transistorwill be discussed in terms of holes and electrons, in a mannerconsistent with the assumption noted above, that the boy 1 is of n-typematerial. That is to say, instead of using the generic terms majoritycarriers and minority carriers, reference will be made to electrons andholes, respectively. It is considered that the use of this more specificterminology will make the explanation more concise and more readilyunderstandable. It should be understood, however, that it is notintended, by the use of this terminology, to limit the invention ton-type semi-conductive material.

In any transistor, the hole current flowing from the emitter to thecollector produces a concomitant electron current flowing from thecollector to the base. The collector cuirent is the sum of the holecurrent and the electron current, and is therefore greater than the holecurrent by a factor commonly designated (1* or a and termed theintrinsic current amplification at the collector, or sometimes calledthe collector multiplication factor. Due to an amplifying mechanismwhose exact nature is not material to the present discussion, each holereaching the collector may produce a flow of several electrons from thecollector, so that the intrinsic current amplification may be very high.

It has been found by experiment that when a single emitter is placed ata point substantially half-way between two similar collectors, thecurrent carriers from the emitter do not divide equally between thecollectors, as might be expected, but instead most of the current tendsto go to one collector or the other. It is considered that thisphenomenon is due to a cumulative effect. To understand this effect,consider that in any physical transistor it is a practical impossibilityto place an emitter electrode exactly half-way between two collectors;likewise, it is impossible to bias the two collectors with exactly equalpotentials; similarly, it is impossible to give two collectors identicalforming treatments so as to obtain equal current amplification and equalback resistances at both collectors. Consequently, in any physicalset-up, there is always a. slight unbalance in favor of one collector.That slight unbalance produces an unbalance in the electric field withinthe semi-conductive body, which unbalance favors one collector and tendsto attract more than its share of carriers from the emitter. If thetotal emitter current is constant, this additional emitter currentflowing to one collector must be taken away from the unfavoredcollector. These changes in hole currents flowing to the collectors inturn produce changes in the electron currents flowing away from thecollectors. If the intrinsic current amplification (o8 or a at eachcollector is big enough, then these changes in electron current overcomeany changes in conductivity due to the injected holes and the electricfields emanating from each collector will change in the same directionsas the electronic currents. Thus, the favored collector will have alarger field emanating from it and the unfavored collector a smallerfield.

These changes in field will tend to produce a further unbalance in thedivision of the incoming emitter current. This in turn will produce achange in the fields emanating from the collectors, and so forth.

In the transistor illustrated in the drawing, the situation justdescribed exists with respect to the emitter 3e and the collectors c and60. These two collectors should be made to have characteristics asnearly equal as possible. This symmetrical situation is disturbed by thepresence of the additional emitter 42 located nearer the collector 60than is the emitter 3e. The bias potentials on the two collectors, orother controllable factors, should be selected so that when there is nocurrent flowing through the emitter 42, most of the holes from theemitter 3e flow to the collector 50, with a small current flowingthrough collector 60. As soon as a current starts to flow from theemitter 4e, it is attracted to the collector do by the electric field ofthat collecor. This hole current, as it arrives at the collector,increases the electron current from the collector in a ratio determinedby the intrinsic amplification factor of, previously mentioned. Thisincreased electron flow increases the electric field of collector 60,which increased field attracts more of the holes from emitter 3e, andthe increased flow of holes further amplifies the electron flow fromcollector 66.

This attraction of holes from the collector 3e by the electric field dueto the electron current flowing from emitter 4e may be described as aninternal feedback mechanism. This mechanism is similar, if notidentical, to that present in the transistor disclosed in the Rutzapplication previously mentioned.

The present transistor has, however, another internal feedback mechanismoperating which further enhances its overall current amplificationfactor. Referring to the collector 50, it may be seen that as theproportion of holes from emitter 3-2 flowing to this collector getssmaller.

the electron current flow from collector 5c is reduced, the electricfield due to that electron flow decreases and collector 5c consequentlytends to receive an even smaller proportion of the holes from emitter3e.

It may therefore be seen that a transistor constructed as describedabove has an overall current amplification factor even greater than thatof the Rutz type of transistor. Furthermore, it has been observed thatthe back resisatnce of the transistor described above is maintainedbetter than is the back resistance of the Rutz transistor.

It should be further noted that the transistor described herein is notsubject to any limitation with regard to the lifetime of the excesscarriers in the semi-conductive ma terial, nor as to the diffusionlength. The electrode spacings should, on the other hand, be smallerthan the diffusion length, rather than greater as in the Rutztransistor.

It is desirable to keep a high bias voltage on the emitter 3e, and ahigh resistance in series with it so that there is a plentiful, butconstant, supply of current carriers continuously available from thatemitter. The bias voltage on the emitter 3e and that on the collector Scmay be made variable, as indicated in the drawing, in order that thecurrent amplification factor can be controlled as desired.

While a resistor 12 is shown in series with battery 11, this resistormay in many cases be omitted, and indeed it is preferable to omit it. Itmight be expected that the best balance between collectors 5c and 6cwould be obtained by making resistor 12 equal to resistor 13 and thepotential of battery 11 equal to that of battery 14. However, asexplained above, an accurate balance between the emitters cannot bepractically attained, and is not in fact utilized in the operation ofthe transistor, except as a transitional condition during a transferfrom an unbalance favoring one collector to an unbalance favoring theother. The desired operation then is to pass as quickly as possible fromone condition of unbalance to the other. Resistors 12 and 13, like allresistors, have a tendency to maintain the current flow through themconstant, and therefore tend to be detrimental to the desired operationof the circuit. The load resistor 13 must be retained in the circuit,but resistor 12 may readily be omitted. If resistor 12 is omitted, thenin order to obtain nearly equal voltages at the collectors 5c and 6c,the potential of battery 14 should be made greater than the potential ofbattery 11, in order to compensate for the potential drop acrossresistor 13.

While I have shown and described a preferred embodiment of my invention,other modifications thereof will readily occur to those skilled in theat, and I therefore intend my invention to be limited only by theappended claims.

I claim:

1. A current multiplication transistor circuit, comprising a transistorhaving a body of semi-conductive material of uniform conductivity type,a first electrode in ohmically conductive contact with one side of saidbody, second, third, fourth and fifth electrodes in asymmetricallyconductive contact with the opposite side of said body, a load resistorand a first source of unidirectional electrical energy connected inseries between said first and second electrodes, said source being poledto reversely bias said second electrode, a second source ofunidirectional electrical energy, means connecting said second sourcebetween said third electrode and said first electrode, said secondsource being poled to bias said third electrode reversely, said fourthelectrode being located substantially equally distant from said secondand third electrodes, a third source of unidirectional electricalenergy, means connecting said third source between said first and fourthelectrodes, said third source being poled to bias said fourth electrodeforwardly, said fifth electrode being located closer to said secondelectrode than said fourth electrode, and a source of variable inputsignal potential connected between said first and fifth electrodes, saidthird source of energy cooperating with said fourth electrodecontinuously to introduce minority current carriers into saidsemi-conductive body, said first and second sources biasing said secondand third electrodes so that they competitively attract said minoritycarriers, said second and third electrodes and their respectiveconnections being unbalanced in favor of said third electrode so that ittends to attract the greater proportion of said carriers, said signalsource being operable at times to inject additional minority carriersinto said body, said second electrode being thereupon effective toattract substantially all said additional carriers, the flow of saidadditional carriers being effective to modify the electric field in saidbody to reverse the unbalance in favor of said third electrode and toestablish an unbalance in favor of said second electrode, therebyfurther increasing the minority carrier flow to said second electrodeand thereby greatly increasing the current multiplication at said secondelectrode, and output means connected to said second electrode.

2. A current multiplication transistor circuit as defined in claim 1, inwhich said means connecting said second source between said thirdelectrode and said first electrode comprises direct connectionssubstantially without impedance.

3. A current multiplication transistor circuit as defined in claim 1, inwhich said means connecting said third source between said fourthelectrode and said first electrode comprises high impedance meanseffective to maintain the current flow through said fourth electrodesubstantially constant.

4. A current multiplication transistor circuit as defined in claim 1, inwhich at least one of said second and third sources is variable inpotential to control said increased current multiplication.

References Cited in the file of this patent UNITED STATES PATENTS2,592,683 Gray Apr. 15, 1952 2,655,607 Reeves Oct. 13, 1953 2,660,624Bergson Nov. 24, 1953 2,679,619 Grassl May 25, 1954

